This expression is idly thrown about nowadays. It implies that a solution exists and can't be further improved upon. Just because a solution exists, or is the current fashion, doesn't mean it's a 'solved problem'. Beer brewing is a 'solved problem'.. yet there are some brews you couldn't pay me to drink. Interesting problems are never fully solved, they are simply iterated on over and over. Payment processing was a 'solved problem' until Stripe came along. Assembly line manufacturing of cars was a 'solved problem' until Toyota showed Ford how it could be done better.